The solubility of most ions in solution decreases with a decrease in solution temperature. If dissolved ions are present near their saturation concentration in the solution, a significant reduction in the temperature of the system can result in supersaturation and precipitation of a portion of these supersaturated ions. Precipitates can combine and deposit as a scale on any solid surface with which they come into contact, such as the vessel or conduit in which the solution is confined.
One example of such a solution are certain high-enthalpy or high temperature liquids produced from a geothermal well, i.e., a high temperature geothermal brine from a liquid-dominated reservoir. As discussed by Bowen and Groh ("Energy Technology Handbook," D. M. Considine, Editor, at page 7-4 of Chapter 7 entitled "Geothermal Energy"), liquid-dominated reservoirs may be conveniently divided into two types: one type having high-enthalpy fluids above 200 calories/gram; and one having low-enthalpy fluids below this point. High temperature type brines have been defined by in-situ reservoir temperatures, the high temperature type having in-situ temperatures generally above 180.degree. C., typically above 200.degree. C., most commonly above 220.degree. C., and the low temperature type having temperatures below these values. The high temperature or high-enthalpy brines especially tend to dissolve reservoir rock or contacting solids and these brine types contain dissolved solids in concentrations ranging from around 2,000 to as much as 260,000 ppm by weight.
An especially troublesome dissolved solid component of the high-enthalpy brine is silicon, which may be found at or near saturation concentrations in the form of silicic acid oligomers. These tend to precipitate out at almost every stage of brine processing as the temperature is lowered, either as substantially pure silica or as a tightly adherent metal-silica/metal-silicate scale. Unless inhibited, naturally occurring silica-rich scale/precipitation (as the brine is cooled) must be removed frequently. This precipitation tendency and removal need is especially true as lower brine temperatures are reached during the cooling process.
In order to extract thermal energy from a liquid geothermal brine, the brine temperature is reduced. Heat exchangers are commonly used for low-enthalpy brine applications, such as producing hot water. The brine's thermal energy is transferred within the heat exchangers to the hot water. The heated water may in turn heat air (for space heating) or other fluids. Even though the low enthalpy brines may be saturated with dissolved solids, the limited amount of temperature reduction possible for these low-enthalpy (i.e., moderate temperature) brines produces little or no precipitation and fouling of heat exchange surfaces or plugging of injection wells. This lack of significant precipitation or fouling is also believed due to the relative stability of slightly supersaturated brines. Even if the supersaturated brine is not stable, the low precipitation rates (i.e., slow precipitation kinetics) at the moderate brine temperatures within these heat exchangers are also believed to inhibit large amounts of precipitation and fouling.
However, high-enthalpy or high temperature brines typically have larger saturation concentrations of dissolved solids and faster precipitation kinetics. Larger amounts of heat removal can also produce significant levels of supersaturation. High enthalpy brines therefore tend to produce copious quantities of scale which would quickly foul a conventional heat exchanger. Thus, conventional heat exchangers are not generally employed for high-enthalpy brines, even though extraction of heat from such brines using a heat exchanger process may otherwise be beneficial.
Other methods for extracting energy from high-enthalpy brines are commonly used because of conventional heat exchanger fouling. One such method is flashing, which is accomplished in a vessel where brine pressure is reduced. As a result, a portion of the brine is flashed to steam and other gases while the temperature of the residual brine is decreased. Flashing is often accompanied by massive amounts of precipitation formation that may scale and eventually plug piping. Other processes which avoid a fouled heat transfer surface, such as total flow and direct-contact (fluid-to-fluid) heat exchange processes, have also been proposed for high-enthalpy brines.
Because of massive scaling, various proposals have been made to decrease the scale formation in flash or other non-heat exchange surface equipment used in producing and handling high-enthalpy geothermal brines. In "Field Evaluation of Scale Control Methods: Acidification," by J. Z. Grens et al, Lawrence Livermore Laboratory, Geothermal Resources Council, Transactions, Vol. 1, May 1977, there is described an investigation of the scaling of turbine components wherein a geothermal brine at a pressure of 220 to 320 p.s.i.g. and a temperature of 200.degree. to 230.degree. C. (392.degree. to 446.degree. F.) was expanded through nozzles and impinged against static wearblades to a pressure of 1 atmosphere and a temperature of 102.degree. C. (215.degree. F.). In the nozzles, the primary scale was heavy metal sulfides, such as lead sulfide, copper-iron sulfide, zinc sulfide and cuprous sulfide. Thin basal layers of fine-grained, iron-rich amorphous silica appeared to promote the adherence of the primary scale to the metal substrate. By contrast, the scale formed on the wearblades was cuprous sulfide, native silver and lead sulfide in an iron-rich amorphous silica matrix. When the brine which originally had a pH of 5.4 to 5.8 was acidified with sufficient hydrochloric acid to reduce the pH of the expanded brine to values between 1.5 to 5.0, scaling was dramatically reduced or eliminated.
However, such acidification, especially at a pH near 1.5, tends to significantly increase the corrosion of the brine-handling equipment. If a heat exchanger were to be used to handle strongly acidified brines, added wall thickness or excessively costly materials of construction would be required. If added wall thickness heat exchangers are used, frequent removal of corrosion products from the heat exchange surfaces may also be required.
Still further, strong acid treatments can cause other geothermal fluid handling problems. These can include the introduction of oxygen into the otherwise oxygen-free brine, embrittlement of equipment, and injection formation problems. Thus, commercial acid treatments of geothermal brines known to the inventors are limited to small changes in pH. This accepts the residual amount (not the complete elimination) of scale, especially silica, deposited on flash process equipment in return for acceptable corrosion rates and significant reductions in scaling rates. Reducing scale formation decreases the amount of scale removal, but deposits would still quickly foul a heat exchange surface making a heat exchange process impractical without very frequent cleaning.
While the aforementioned acidified geothermal brine and acidified brine plus reducing agent treatments have met with some success in some non-heat exchanger surface applications, the need exists for a further improved treating process to further decrease fouling of a heat exchanger. Controlling fouling tendencies in materials commonly used in heat exchangers without significant added cost would allow economic energy extraction from some high-enthalpy brines. The economic advantages of being able to extract energy in a heat exchange process is especially beneficial when the high enthalpy or high temperature brines contain high dissolved gas contents, avoiding the need for costly non-condensible gas removal equipment normally required for a condensing flash process.
Accordingly, it is an object of this invention to provide an improved method for decreasing or virtually eliminating the overall precipitation and scaling of these brines, particularly silica and iron-silicate scale, so as to prevent significant fouling of heat exchanger surfaces. It is also an objective of this invention to control corrosion of heat exchanger surfaces composed of common used materials of construction, such as low carbon steels.
Other objects, advantages and features of the invention will be apparent from the following description, drawings and appended claims.